Global warming is now an issue of worldwide concern, and the greenhouse effect caused by carbon dioxide and methane gas plays a significant role in global warming. Global warming not only disturbs the ecosystem but also has a huge impact on human social life. In this regard, efforts are being made in various aspects to reduce atmospheric emissions of carbon dioxide and methane gas.
In sewage treatment plants, waste water treatment plants, landfill sites, etc., organic substances contained in waste generate gas through decomposition. This gas is called landfill gas. At the initial landfill stage, the landfill gas is decomposed in the presence of oxygen. However, as oxygen is gradually reduced, the landfill gas is primarily decomposed in an anaerobic digestion process. Most of the landfill gas generated in the anaerobic digestion process contains 40 to 60% carbon dioxide, 45 to 60% methane gas, and very small amounts of other components such as nitrogen and ammonia. Methane and carbon dioxide, which are main components of the landfill gas, are the causes of global warming. To make the landfill gas industrially applicable, methane gas and carbon dioxide should be separated from each other.
Global warming caused by an increase in carbon dioxide in the air is one of the important environmental problems that must be solved by mankind. Carbon dioxide is emitted from sewage treatment plants, waste water treatment plants, landfill sites, etc. when waste materials are burned. Carbon dioxide is particularly a problem when emitted from thermoelectric power plants or steel mills. Therefore, technologies of separating and removing carbon dioxide from generated waste gas are being developed. Some carbon dioxide separation technologies already developed include an absorption method, an adsorption method, a cryogenic air separation method, and a membrane separation method.
The absorption method is a method of selectively separating carbon dioxide by absorbing carbon dioxide. In the absorption method, a combustion or process gas that contains carbon dioxide is brought into contact with a solution, such that carbon dioxide can be absorbed by a chemical reaction. Of the absorption method, a wet amine method is commercially available technology. In the wet amine method, carbon dioxide is collected from a combustion exhaust gas using an amine-based absorbent.
The adsorption method is a method of separating carbon dioxide by making carbon dioxide be physically adsorbed onto the surface of an adsorbent having affinity for carbon dioxide.
The cryogenic air separation method is a classic gas-liquid separation method for separating carbon dioxide liquefied at low temperature from other gases not liquefied. This method is advantageous in that it can produce a large amount of liquefied carbon dioxide but disadvantageous in that it requires a lot of energy for cooling.
The membrane separation method generally uses a solid membrane having a separation function. The membrane separation method is widely usable from the molecular level to the particle level depending on the type of membrane used. In addition, since a material is usually separated using pressure which is mechanical energy, less energy is consumed in the membrane separation method than in a distillation method using thermal energy. Applied examples of the membrane separation method include reverse osmosis, ultrafiltration, precision filtration, dialysis, and gas separation. In particular, the gas separation method is drawing attention as a method of separating and collecting carbon dioxide in an energy-saving manner from large-scale sources of carbon dioxide such as thermoelectric power plants, cement plants, and steel mill furnaces.
More specifically, a gas separation membrane that can be used in the membrane separation method to separate and collect a particular gas from, e.g., natural gas, may be an aromatic polyimide membrane which is obtained by polymerizing and imidizing an aromatic tetracarboxylic acid component and an aromatic diamine component. Research has been actively conducted on the aromatic polyimide membrane. However, the aromatic polyimide gas separation membrane can be manufactured only at a high temperature of 350 t or above and has problems in heat resistance, durability, and chemical resistance. Therefore, solutions to these problems need to be researched.
Other conventional technologies of separating and collecting carbon dioxide are disclosed in Korean Patent Publication No. 10-0734926 and Japanese Patent Laid-Open Publication No. hei 10-180062. Korean Patent Publication No. 10-0734926 discloses an apparatus for removing a sulfur compound and separating methane and carbon dioxide using a liquid iron chelate catalyst. The apparatus can process a sulfur compound in a bad-smelling gas generated by a landfill site or an anaerobic digester and separate and collect methane and carbon dioxide in the gas. In addition, Japanese Patent Laid-Open Publication No. hei 10-180062 discloses a separation membrane and a selective separation method. Here, the separation membrane can separate carbon dioxide from a mixture of carbon dioxide and methane using a dense membrane or an asymmetric membrane that contains, as its main component, fluorine-containing polyimide resin having high separability and permeability for carbon dioxide.
Until now, various methods of separating and collecting carbon dioxide including the above conventional technologies have been suggested. However, it is difficult to form a separation membrane having a large size of 10 cm2 or more. In addition, when partial pressure on both sides is used, a massive amount of energy is consumed, and a separation membrane that can withstand this pressure difference cannot be formed.